In general, a frequency control ancillary service (FCAS) is a service for maintaining a power grid frequency during a normal operating state and a contingency event (such as generator outage or a failure in a transmission line) within a range of 60±0.2 Hz defined in the electricity law. The FCAS can be classified into a frequency response ancillary service or a primary frequency control, a frequency regulation ancillary service or a secondary frequency control, an economic dispatch or tertiary frequency control depending on a response time.
In this regard, the FCAS tries to contribute to maintaining a proper frequency using an automatic generation control (AGC) in order to allow correction of an error that may be generated due to imbalance between supply and demand unlike a normal operating state to comply with the dispatch schedule, correction of a difference in a generation capability to comply with the dispatch schedule, fast frequency recovery in a contingency event.
The frequency response ancillary service may be provided by a governor of a generator or a contracted interruptible load in order to suppress abrupt frequency reduction in a power grid system in a particular contingency event or maintain the lowered frequency state for a certain time.
Korean patent application publication No. 2013-0102681 discusses a technique in which how many energy storage systems are available and whether or not an energy storage system is rejected are determined by setting priorities of energy storage systems depending on the state of charge (SOC) of the energy storage system, discharging is performed when the frequency supplied to a system load decreases, and charging is performed when the frequency supplied to the system load increases. However, this technique of the related art is not applicable to the AGC or the frequency control operating at a standard frequency of the power grid frequency control market and a frequency dead band disadvantageously.